#include <bits/stdc++.h>
using namespace std;
int main()
{
    int a, t = -1;
    cin >> a;
    for (int i = 8; i < a; i++) {
        if (a / i == 1 && a % i == 7) {
            t = i;
            break;
        }
    }
    cout << t << endl;
    return 0;
}